1. Field of the Invention
This invention relates to data information and processing systems employing a central station and a plurality of input and/or output terminals connected to the central station by a communication channel and more particularly to such systems wherein some of the terminals have a data acceptance and/or transmission rate which differs substantially from that of other terminals.
2. Reference to Related Patent Applications
U.S. Pat. application Ser. No. 345,945 entitled "Computer System", filed Mar. 29, 1973, now U.S. Pat. No. 3,821,706 discloses a remote terminal computer system which could be adapted to employ the concepts of the present invention.
3. Prior Art
Computer terminals having the ability to input and/or output data to a remotely located central computer station are commonly employed in commercial and industrial establishments such as banks and factories and in wired television program distribution systems such as CATV. A wide variety of input/output devices are available for use with these terminals ranging from simple pad type keyboards of the type used with touch-tone telephones to CRT displays with input storage and forwarding capabilities. These terminals differ widely in data reception and transmission capability: a teletype printer may have a maximum data reception rate of 110 bits per second while a CRT terminal is capable of accepting or transmitting data at rates in excess of several thousand bits per second. In previous multiple terminal systems employing terminals of such desperate data rates it has been the practice to provide a single forward data channel for messages from the central computer to the terminals having a sufficient capacity to carry data at the rate of the highest speed terminal on the line.
In those multiple terminal systems in which the terminals have the capability of addressing the central station, these return communications typically are carried over a single channel sequentially and means are provided for granting various terminals, having messages to be sent to the central station, access to the channel in some ordered manner. In one previously utilized class of systems this was achieved by the central station sequentially addressing the terminals over the forward channel and inviting them to return short messages to the central station over the return channel. This arrangement is relatively wasteful of forward channel capacity and in the aforementioned patent application a system is disclosed wherein each terminal has a unique address and contains a counter advanced by a single count each time a digital word is transmitted by the central station on the forward channel. When a terminal's counter reaches a count equal to its address the terminal may then provide a short message to the central station on the return channel if the terminal has a message to be returned. By this arrangement the terminals are "polled" at a rate controlled by the rate of transmission of digital words by the central station. The addition of a high data rate terminal, such as a CRT, in a system which largely comprises low speed terminals such as teletypes, dramatically increases the polling rate in a system of this type. Therefore, with a single channel for both low and high speed devices, there is a severe restriction on the total number of devices which can be rapidly serviced.